In a pan, melt butter. Add the diced onions and sauté for about 2 minutes. Add the grated garlic and cook for another 2 minutes.
Add salt, black pepper, red chili flakes, oregano, and mixed herbs. Stir well. Then add the chopped coriander. Cook until the onions turn golden brown and soften.
Pour in the milk and stir continuously until the mixture comes to a gentle boil.
Add the cheese slices or grated cheddar and stir until melted and smooth.
Transfer the caramelised onion mixture to another bowl and let it cool for about 30 seconds.
Toast the bread slices. Spread a layer of the caramelised onion spread over each slice.
Sprinkle with chopped coriander and red chili flakes before serving.